I use DropBox as well Blossom. Unless we're doing it a different way. After I download the file, I copy it and paste it in the Books folder. Then click on it. It prompts you to choose an app to open with. Select Amazon Kindle. The reader app doesn't open, but when you go into 'Books" it's there.

I'm not getting prompted for that can you give me more step by step instructions I must be missing something.